Can you believe that just happened? I’m still rubbing my eyes (probably a bad idea because it’s flu season). No one expected, well, quite this: Valve hopping into the operating-system game, training — and make no mistake here — its sights on Microsoft, Sony, Apple, Google and anyone else I’m forgetting in the non-Unix-like-OS column.

“Thousands of games, millions of users. Everything you love about Steam. Available soon as a free operating system designed for the TV and the living room,” writes Valve on the new SteamOS promo page, which unlocked at exactly 1:00 p.m. E.T. on Monday. The picture above, probably a Photoshopped purplish negative of the sun, shows SteamOS dangling in orbit like an alien satellite, presumably the first of others to come. We have at least two more reveals, with the final one possibly the storied Steam Box everyone’s been talking about for ages.

Make no mistake, SteamOS is meant to be a living-room operating system, says Valve. Make of that what you will, but probably not something that goes on a smartphone (at least not at this point). But something you’d load on a Valve-branded tablet? Don’t laugh. And a home-brew PC? Perhaps. Valve’s pitching SteamOS as “available soon as a free stand-alone operating system for living-room machines,” machines being plural, thus casting doubt on notions it’ll be exclusive to proprietary hardware (assuming that’s even in the offing). And maybe a posttablet device no one’s seen yet? Never say never, because Valve honcho Gabe Newell has already spoken of such things.

While we’re waiting for the next timer to zero out, let’s run through SteamOS’s salient features.

It’s basically still Steam.

Or that’s what it sounds like, based on Valve’s description: “Finally, you don’t have to give up your favorite games, your online friends and all the Steam features you love just to play on the big screen,” writes the company. Think thin client, like Google’s browser-driven Chrome OS — just the basics, because that’s all you really need in the living room.

But now with better graphics processing.

So sayeth Valve, anyway, noting it’s “achieved significant performance increases in graphics processing, and we’re now targeting audio performance and reductions in input latency at the operating-system level.” Valve says game developers — just “game developers,” no names yet — are already availing themselves of these performance perks “as they target SteamOS for their new releases.”

It’s also … modular?

Valve described SteamOS as a “cooperating system,” in which “each participant is a multiplier of the experience for everyone else.” No, probably not in the sense you might be thinking, à la SETI@home, where each user’s processing cycles contribute to some grand process, but metaphorically speaking. This sounds like Valve’s way of saying it plans to make the experience more collaborative, say, than the experience you’re used to having with Apple’s App Store, Microsoft’s Xbox Live or Sony’s PlayStation Network.

According to Valve:

With SteamOS, openness means that the hardware industry can iterate in the living room at a much faster pace than they’ve been able to. Content creators can connect directly to their customers. Users can alter or replace any part of the software or hardware they want. Gamers are empowered to join in the creation of the games they love. SteamOS will continue to evolve, but will remain an environment designed to foster these kinds of innovation.

Treat all of that as lofty rhetoric until we get specifics, of course, but in theory … well, at least I’m intrigued.

You can stream games from your Windows or Mac system to a SteamOS machine.

The strongest argument that a Steam Box is imminent: Valve’s going to let you pipe existing Steam games from PCs around your house, over your home network, straight to your SteamOS machine in the living room. Latency-free (or near enough)? We’ll see.

You can also stream “music, TV, movies.”

Valve says it’s “working with many of the media services you know and love.” Spotify? Netflix? Hulu? Rdio? Amazon? Who knows. Presumably those and many more, assuming — and I think this is a safe assumption — Valve wants to compete in the Roku/Apple TV category.

Plus “Family Sharing,” which we already knew about.

Announced a few weeks ago, this is Valve’s intrepid plan to let you fold in up to 10 friends and family members (you pick), each of whom can access a game you’ve paid for, earn their own achievements and save their own games.

And last but not least, “Family Options.”

Don’t want to see what your parents are playing? Want to tweak your Steam library distinct from someone else’s, using the same SteamOS interface? Valve promises “families will have more control over what titles get seen by whom, and more features to allow everyone in the house to get the most out of their Steam libraries.”

And now we wait for the second of three announcements to drop on Wednesday at 1 p.m. E.T. If SteamOS — unexpected, bold and at least theoretically a very big deal — was just the first, imagine what’s on deck next.